1. Field of the Invention
This invention relates to apparatus and methods for extruding polymer compositions.
2. Introduction to the Invention
Polymeric compositions are often shaped by melt extrusion. A conveying means conveys the molten polymeric composition as a continuous stream under pressure to an extrusion orifice, and forces it through the orifice to form an extrudate of desired shape. In many cases, the composition is fed as a solid to a conveying means which melts the composition through a combination of shearing and external heating. Well known conveying means include single screw and twin screw extruders. Unfortunately, conventional conveying means often fail to deliver the molten composition to the extrusion orifice at a constant rate, as a result of pressure variations within the conveying means, particularly when the composition has high viscosity. This results in an extrudate of variable quality. It is well known to mitigate this problem by placing a gear pump (alternatively known as a melt pump) between the conveying means and the extrusion orifice. The gear pump helps to deliver a substantially constant volume of molten polymeric composition to the extrusion orifice, and in consequence to produce an extrudate of improved quality. However, the gear pump must be correctly sized and the pressure variations must be small enough to ensure that the gear pump is neither starved nor overfed.